Providing power to circuitry in an integrated circuit (via turning on an internal voltage rail) can be a difficult proposition, especially during power-on (turn on) and power-off (turn off). For example, during power-on, when power is initially provided to the circuitry, the power-on current can increase rapidly and result in a large transient current. Large transient currents can be damaging to the transistors in the circuitry. This problem can be further exacerbated when high-gain transistors, such as input/output (I/O) PMOS transistors, are used as the switch for the power supply. In such a situation, the fast turn on could result in large transient currents while these high-gain transistors charge up the internal rail.
One commonly used technique to help reduce the magnitude of the transient current is to use multiple switches arranged sequentially with respect to one another and coupled in between the power supply and the circuitry. Each of the switches in the sequence of switches can be smaller than the single switch that they replace. Then, the switches can be turned on in single fashion, with a small delay between consecutive switches. Since each switch is smaller than the switch that is replaced, the amount of current is smaller. Furthermore, with a delay being present between consecutive switches being turned on, the transient current is spread out over time. This can effectively reduce the severity of the transient current.
One disadvantage of the prior art is that even with smaller transistors being used in the sequence of switches, as each switch is being turned on, a transient current of significant magnitude can still occur. Therefore, with the use of multiple switches, a sequence of transient currents can be produced, each having a magnitude that can cause problems.
Another disadvantage of the prior art is that since the large transient current will typically occur rapidly after the switch is closed, it can be desirable to accelerate the power ramp-up after the danger of the transient current has passed and the use of the sequence of switches does not allow for the acceleration of the power ramp-up.